1. Field of the Invention
The present invention relates to a printhead substrate, printhead, and printing apparatus.
2. Description of the Related Art
There is known a printing apparatus which prints information such as a text or image on printing medium such as paper or a film. Most printing apparatuses of this type adopt a serial printing method of printing while reciprocally scanning in a direction perpendicular to the printing medium feeding direction. The serial printing method has advantages such as easy cost reduction and easy downsizing.
Of these printing apparatuses, a printing apparatus complying with an inkjet printing method (to be referred to as an inkjet printing apparatus) is known. The inkjet printing apparatus includes, for example, a printhead which prints using thermal energy (see Japanese Patent Laid-Open No. 2005-199703).
The printhead has an inkjet printhead substrate (to be simply referred to as a printhead substrate). FIG. 7A is a view exemplifying the circuit layout of a conventional inkjet printhead substrate 500. FIG. 7B is a circuit diagram exemplifying a circuit arrangement for driving an array of heaters 502 on the printhead substrate 500 shown in FIG. 7A.
As shown in FIG. 7A, the printhead substrate 500 includes heaters, transistor drivers, high-voltage logic circuits, logic circuits, and the like. As shown in FIG. 7B, various different driving powers are supplied to the respective portions. To increase the printing process speed and image quality, the printhead substrate 500 tends to increase the number of heaters and elongate the substrate shape.
In general, printhead substrates are promoting efficient circuit arrangements for downsizing and the like. However, the printhead substrate requires an ink supply port 501 as shown in FIG. 7A, which greatly restricts the circuit arrangement, compared to a general IC whose shape is arbitrary. Since the substrate shape tends to be long, as described above, a long circuit arrangement along the heater array is necessary. For a long substrate shape, a long power supply line needs to be laid out from a pad at the end of the substrate. The parasitic C, R, and L of the power supply line may make the power supply unstable, causing a malfunction.
On the printhead substrate, a pulse current of several ten mA per heater or several A per entire substrate flows at once to cause film boiling of ink. The pulse current flows through an aluminum wire running through the entire upper layer of the substrate circuit. Thus, noise superposed on the circuit power supply becomes greatly large, compared to a general IC. In particular, a logic circuit 505 shown in FIGS. 7A and 7B needs to be driven quickly at low voltage. However, under layout restrictions, the logic circuit 505 is arranged parallel to the lower layer of the aluminum wire through which the heater pulse current flows, and thus is greatly affected by noise.
Since the number of heaters increases, as described above, noise tends to further increase. A larger number of heaters require higher circuit driving speed (higher frequency), and the power supply becomes unstable along with an increase in power consumption. Also, high-speed driving increases power supply noise, and the risk of the malfunction of the circuit rises. That is, stabilization of the power supply on the substrate is an important issue.